The present invention relates to an apparatus for regulating video signals produced by scanning an original document which has an optical image pattern thereon.
A copying machine or a facsimile transceiver for instance includes an image input device for scanning an image pattern on a document on a glass platen using a CCD solid state image array or the like. Generally, the level of the video signals produced by such an array varies with the density level of the background of the document as well as the amount of light emitted by illumination lamps. For this reason, regulation of the video signals is necessary.
It has been practiced to detect the peak level of the video signals, or the density level of the background of the document, and regulate the video signals while variably setting an image signal level which is about 50% of the detected peak level. However, this regulation process is effective only on a two-value basis and cannot cope with fluctuations in the amount of light from the lamps or changes in the background density level when regulation is desired on a multi-value basis.
For multi-value regulation of video signals, there has been employed a process wherein given image signal levels of different densities are determined in advance and a sensor senses the amount of light output of the lamps so that each of the image signal levels may be suitably modulated in accordance with the sensor output to compensate for changes in the amount of light output. This process, however, cannot cope with variations in the density of the background area of the document. Since the background level differs a great deal from documents of one kind to those of another kind, the process mentioned above tends to erroneously determine the background of one document as an image area when regulating the video signals.
In the present type of apparatus, it is generally practiced to remove a background density level from the video signals so that an image free from contamination in the background areas may be reproduced.
A conventional expedient for the removal of the background is offsetting the reference point of the video signals by the background level expected in advance. However, this expedient fails to readily and properly remove the background in for a document whose background density may vary over a wide range. Particularly, where a document has an uneven density distribution in its background area, the expedient mentioned above cannot follow the difference in density and results in irregularity in the removal of the background whereby the background area of a reproduced image is partly contaminated, degrading the quality of the image.
Video signals read by a CCD or PDA solid state scanning element through photoelectric conversion corresponding to density information have traditionally been processed into two different binary values by simply discriminating the video signals in an analog fashion with respect to a certain slice level. However, a predominant method practiced today for binary processing is to transform the video signals into digital values through an analog-to-digital converter, process the digital signals in various ways using multi-value conditions, and then convert the digital signals into binary form.
Conventionally, a white level (zero) reference and a black (maximum value) reference have been determined for the analog-to-digital conversion of video signals by detecting the zero value and peak value of the video signals (as disclosed in Japanese Unexamined Patent Publication No. 55-5582). This system involves problems as will be discussed hereinafter. Where a document such as printed matter has handwritten characters which are different in density from printed characters or hairlines which have high spatial frequencies even if of the same density level, the photoelectric conversion will produce video signals which are uneven in output level as indicated in FIG. 10. When the peak value V.sub.P of the video signals is detected and employed as a black level reference for analog-to-digital conversion according to the quantitizing system mentioned above, an output level V.sub.A corresponding to a hairline and an output level V.sub.B corresponding to handwritten characters will be quantitized individually into different digital values. The result is that, when an image is to be reproduced on the basis of the video signals, the achievable quality of the reproduced image becomes poor.